新竹頭前溪上游集水區地層滑動與植生分布在颱風事件中之關係

The Relationship between Landslide and Vegetation during Typhoon events in the upstream of Tao-Cheng River, Hsinchu

陳麒文(Chi-Wen Chen)

指導教授 ： 陳宏宇

國立臺灣大學/理學院/地質科學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00912

摘要

本研究嘗試探討新竹頭前溪上游流域自1996年至2008年間之地層滑動與植生分佈狀態在6個不同的颱風事件中之關係。從河水取樣分析中發現，本研究區在乾季從每年10月至次年4月的顆粒性碳含量為介於5.57%至6.39%之間，碳濃度為介於0.22mg/l至2.39mg/l之間。在濕季從每年5月至9月的顆粒性碳含量介於1.43%至2.29%之間，碳濃度介於0.41mg/l至5.4mg/l之間。此意義顯示在濕季時，部分地層滑動或地表沖刷之地質材料進入河道中，增加了顆粒性碳的濃度。在高流量高輸砂量時，因為更多的懸浮質加入其中，使得懸浮質中的碳含量下降，顆粒性的碳濃度則相對升高，乾季時則呈現相反的結果。幾次颱風期間，顆粒性碳濃度高達0.7mg/l至23.67mg/l之間，主要是由於大規模的地層滑動將地表植生及土壤大量帶入河道所形成。
從10年來SPOT衛星影像的分析發現，本研究區常態化差異植生指標(NDVI)在颱風事件前為介於0.47至0.63之間，颱風事件後介於0.38至0.46之間。其中又以1996年賀伯颱風前、後之NDVI的差值0.22為最多，而2008年辛樂克颱風前、後的差值0.06為最少，這可能是因為區域內地層滑動面積的增加或大規模的植生崩落所造成。
從各不同颱風期間地層滑動的判釋結果發現，本研究區具有較高的新生率，介於69.98%至83.30%之間，重現率較低，介於10.93%至40.37%之間，崩塌率則介於0.60%至1.29%之間。新生率以2001年桃芝颱風為最高，1996年柯羅莎颱風為最低；重現率以2005年海棠颱風為最高，2001年艾利颱風為最低；而崩塌率則是以2007年柯羅莎颱風為最高，2004年艾利颱風為最低。就本研究區的3個地層，大桶山層、石底層與南莊層的岩石強度與崩塌率的相對關係而言，大桶山層之岩石強度60.52MPa最強，崩塌率最低，NDVI值為最高。此意義顯示，區域內岩石強度較高的地層，其植生狀態較為良好，較不容易發生地層滑動。
從輸砂量及降雨量的統計發現，各颱風事件的總輸砂量及平均日降雨量皆以2004年艾利颱風期間的3.52百萬噸及276.92mm為最高，2001年桃芝颱風期間的0.18萬噸及19.88mm為最低。就地層滑動與崩塌率的相關性而言，降雨量愈大時，輸砂量也愈大，但崩塌率並不一定有相對性之增加。此意義顯示，高降雨時，河流中的高輸砂量並不完全受到現地地層滑動物質的影響，可能是來自過去崩塌堆積物的沖刷，或高流量對河道本身沉積物的刮蝕所導致。

關鍵字

顆粒性碳 ； 常態化差異植生指標 ； 地層滑動 ； 輸砂量 ； 颱風

並列摘要

In this study, we discussed the relationship between landslide and vegetation distribution in 6 different typhoon events from 1996 to 2008. Through analyzing the particulate carbon from rivers, we found that the average percentage of the particulate carbon is between 5.57% and 6.39% and the average concentration of the particulate carbon is between 0.22mg/l and 2.39mg/l in the dry season. In the wet season, the average percentage of the particulate carbon is between 1.43% and 2.29% and the average concentration of the particulate carbon is between 0.41mg/l and 5.4mg/l. This result indicated that in the wet season, some geological material from landslide or surface erosion entered the river, causing the increase of the concentration of particulate carbon. When the scenario of high flow and high sediment discharge happened, more suspended load joined in, bringing about the decrease of the percentage of the particulate carbon in the suspended load. The dry season had opposite result. Among several typhoon events, the concentration of the particulate carbon can reach 0.7mg/l to 23.67mg/l because the huge landslides caused a large number of vegetation and soil entering the river.
With analyzing the different bands of the SPOT satellite in 10 years, the Normalized Difference Vegetation Index (NDVI) in this study area is between 0.47 and 0.63 before typhoon events and between 0.38 and 0.46 after typhoon events. The highest gap of NDVI value (0.22) occurred in Typhoon Herb and the lowest (0.06) occurred in Typhoon Sinlaku. Extreme disparity of NDVI value would caused by the numbers of landslide areas increasing or a huge scale of vegetation collapse.
From mapping the landslide areas, we found that there are higher new generation ratio of landslide, between 69.98% to 83.30%, and lower reactivated ratio of landslide between 10.93% and 40.37%. The landslide ratio is between 0.60% and 1.29%. For the correlation between rock strength and landslide ratio of the three formations in the study area, we can found the highest rock strength of Datonshan formation has the lowest landslide ratio and highest value of NDVI. This result revealed that the formation with higher rock strength, the vegetation will be better and landslides are not easy to occur.
From the statistics of the sediment discharge and rainfall in each typhoon events, there are the highest sediment discharge (3.52Mt) and average daily rainfall (276.92 mm) in Typhoon Aere and the lowest sediment discharge (1.8kt) and average daily rainfall (19.88mm) in Typhoon Taraji. For the correlation between landslide ratio, rainfall and sediment discharge we can found that higher rainfall caused the higher sediment discharge. But the landslide ratio would not increase relatively. This result revealed that when high rainfall occurred, the high sediment discharge is not affected by the landslide instantly but the erosion of the colluvial deposit previously or the high flow scraped the river itself did.

並列關鍵字

particulate carbon ； NDVI ； landslide ； sediment discharge ； typhoon

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